1. Field of the Invention
The present invention relates to a xe2x80x9csolutionxe2x80x9d method for preparing indium tin oxide (ITO) fine powders, and more particularly, to a method for preparing ITO nanometer grade powders using different starting material sources.
2. Description of the Prior Art
The mixture of indium oxide powder and tin oxide powders, or indium tin oxide powders, i.e., the ITO powders are used as raw materials for ITO sputtering targets. To obtain high quality and high density ITO sputtering targets, nanometer grade ITO powders with uniform composition are required.
ITO is known as an N-type semiconductor material that is composed of indium oxide incorporated with tin. When a indium ion is replaced by a tin ion in the lattice, a free electron is released to preserve charge neutrality and therefore the electrical conductivity of ITO is enhanced.
ITO sputtering targets are typically used in the formation of transparent, conductive ITO films. Since ITO films have the properties of high transmittance to visible light and high infrared (IR) reflectivity, they are widely applied in the fields such as solar cells, flat panel displays, photo-detectors, transparent heating devices, anti-static coatings, electromagnetic protection coatings, and transparent electrodes for liquid crystal displays (LCD).
Various methods have been developed to manufacture ITO films. Some of these well-known methods include vacuum coating, magnetic sputtering, chemical vapor deposition (CVD), and dip coating. The sputtering method is most frequently used due to the fact that it is suitable for large area coating and that it has some environmental merits such as low energy consumption and free of toxic substances during sputtering processes.
In a sputtering process, atoms are sputtered by bombarding the target with high-energy ion beams. These sputtered atoms are then deposited or coated on a substrate in a vacuum environment. The properties of the target such as purity, composition, density, microstructure, diameter distribution, etc. will thus determine the quality of the coated films.
ITO targets are manufactured by pressing and molding the ITO powders to a predetermined shape. The solid-state pressing process may be hot isostatic or cold isostatic pressing. After pressing and molding, the green bodies are sintered to produce ITO targets. The ITO powders are manufactured by mechanically mixing indium oxide powders with a predetermined composition ratio by such a method as ball milling known in the art, the mixture powders are then dried and calcined to form ITO powders.
A problem with the above prior art is that the homogeneity of the mixture of the target is limited and hard to control due to such factors as impurities in the starting materials, composition of the starting materials and their particle size distribution. Therefore, ITO thin film properties such as conductivity, transmittance, and adhesion ability to the substrate are affected.
A proposal of forming high-purity ITO ultrafine powders for sputtering targets is known as xe2x80x9csolutionxe2x80x9d method. According to a prior art of solution method, selected metallic compounds are dissolved in water. After some proper treatments known in the art, the dissolved metallic compounds react and polymerize into particles with higher molecular weight and finally form a stably suspended aqueous solution. After removing the solvent of the suspended aqueous solution by heat, high-purity ITO ultrafine powders are obtained.
However, the xe2x80x9csolutionxe2x80x9d method, like the solid-state process mentioned above, still suffers from the inability to precisely control the powder composition. Consequently, there is a strong need to develope an improved method to solve the above-mentioned problems. The fabrication method described in this invention is much simplified and suitable for different starting material sources and industrial mass production.
The primary objective of the present invention is to provide an improved xe2x80x9csolutionxe2x80x9d method for preparing ITO fine powders used to produce sputtering targets.
According to the claimed invention, a solution method for preparing ITO fine powders is provided. The solution method comprises the following steps:
a. Dissolution: dissolve indium compounds and tin compounds in water respectively to form two solution bodies;
b. Mixing: Mix the two solution bodies by stirring;
c. Precipitation: adding proper amount of alkaline reagents to adjust pH value of the admixed solution in step b to an extent that the precipitation completes;
d. Water washing and filtration: Water in the solution processed by step a through c is removed by using filtration method, a white filter cake is then obtained after filtration, wherein the white filter cake is re-dispersed in de-ionized(DI) by stirring, and wherein step d is repeated at least two times;
e. Peptization: the suspended solution after treated by water wash step is then mixed with proper amount of acids to control the pH value of the solution in a proper range, and then stirring to peptize the solution for a determined time period;
f. Drying: the peptized solution in step e is then dried up to obtain white powders of indium tin hydroxide (ITH); and
g. Calcination: the dry ITH powders are then placed into a high temperature oven, wherein under proper clacining conditions such as temperature heating rate, calcination temperature and calcination time, ITO powders are obtained.
It is to be understood that both the forgoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. Other advantages and features of the invention will be apparent from the following description, drawings and claims.